Outline

Background: A compact proton therapy unit, Monarch250 (Still River Systems, Littleton, MA), is under development. Unlike other large proton therapy units, the entire system including the cyclotron will be housed in a single room, which demands a smaller cyclotron as well as a more compact beam shaping system. Since it is the first such unit in the world, the beam characteristics are not completely known. The aim of this study is, using detailed Monte Carlo modeling, to generate dose profiles, evaluate dosimetric characteristics and treatment plans for the compact proton therapy unit.

Materials and methods: Monarch250 utilizes a super-conducting high-field magnet synchrocyclotron and a passively-scattered beam shaping system (BSS). The protons exit the cyclotron and enter the BSS through a vacuum window approximately 2 meters from isocenter, pass through a first scatterer, a range shifter, a scattering power-compensated range modulator wheel, a contoured and range-compensated second scatterer, a primary and a backup dose monitor chamber, an applicator, and a final collimating aperture.

The MCNPX code system was used to model the proton therapy unit and simulate the beam profiles. We modeled proton source coming out of the cyclotron as a Gaussian distribution with a mean energy of 250 MeV. We further modeled the beam shaping system in detail according to the design data provided by the manufacturer. Spread out Bragg peaks were obtained by simulating dose profiles at different steps of the range modulator wheel and combining the simulation results. Simulations were performed in a water phantom as well as in air. Neutron dose equivalents were calculated based on simulations of neutron fluence in air with a closed aperture. Simulated beam data were used to configure a treatment planning system (TPS). Up to 108 proton histories were tracked per simulation to achieve <2% statistical uncertainties.

Results: A complete set of beam profiles were generated for each of the 14 options for the compact proton unit. The simulated beam profiles for a sample option were shown in Figure 1 [Fig.Â 1]. Beam characteristics such as range, effective SAD, flatness, symmetry, penumbra, spread-out Bragg peak, and neutron dose equivalent were evaluated (Table 1 [Tab.Â 1]). A commercial TPS was configured using the simulated dose profiles. Proton treatment plans were created and evaluated.

Conclusions: The Monte Carlo tool allowed us to evaluate beam characteristics for a compact proton therapy unit prior to its construction. Most beam properties were meeting specifications with a few exceptions such as range discrepancies, which were fed back to manufacturer to optimize the machine design. The TPS generated acceptable proton treatment plans and allowed us to train dosimetrists and physicians. The MC data provided reference data for future acceptance testing and commissioning. These applications will greatly reduce the overall time and cost to commission the machine for clinical use.